S1E12 · Fire

In Bosham, England, a wealthy elderly man says goodbye to his wife before leaving for work, but suddenly catches fire in an apparent case of spontaneous human combustion. His family and house staff—including his Irish gardener, Cecil L'Ively—watch as he burns to death on his front lawn.

Later, in Washington, D.C., agents Fox Mulder and Dana Scully are met by Phoebe Green, an investigator from London's Metropolitan Police and Mulder's former lover from the University of Oxford. Green explains that a serial arsonist is targeting the British aristocracy, burning his victims alive while leaving no trace of evidence. The only links between the crimes are the suspect's love letters to the victims' wives. His latest target is Sir Malcolm Marsden, who is visiting Cape Cod for protection after escaping an attack by the killer. Mulder and Scully visit a pyrotechnics expert who says that only rocket fuel can burn hot enough to destroy evidence of its origins.

Mulder tells Scully that Green, with whom he had a complicated relationship, is using the case to play a mind game, exploiting his debilitating fear of fire. Meanwhile, L'Ively—having killed a caretaker and assumed his identity—greets the Marsden family as they arrive at their Cape Cod vacation home, faking an American accent. Unbeknownst to the Marsdens, L'Ively is painting a layer of rocket fuel onto the exterior of the house. L'Ively befriends the Marsdens' sick family driver, offering to go into town to get him some cough syrup. While there, he uses his pyrokinetic abilities to burn down a local bar purely for fun.

At the hospital, Mulder and Green interview a witness to the bar fire, who tells them of the assailant's apparent ability to will fire into existence. The Marsdens' driver becomes even sicker due to the poisoned cough syrup provided by L'Ively. Because of his illness, L'Ively is recruited to drive the family into Boston that night to attend a party at a luxury hotel. Mulder flies up to Boston to watch over the party with Green, hoping to set a trap for the suspect; Scully continues working on compiling a criminal profile of the killer.

Mulder and Green dance during the party and afterwards kiss; Scully arrives at the hotel and sees them. She also spots L'Ively in the lobby, watching her. A fire alarm goes off after a blaze starts in the Marsdens' room, where the children are located. Mulder attempts to rescue them but is overcome both by his phobia and the intense smoke; they are instead "saved" by L'Ively. When Mulder awakens, Scully questions "Bob" but is dismissed by Green, who believes the man is a long-time employee whose background checks out. Green tells Mulder that she will leave a few days after the Marsdens when they return to England the next day.

Scully discusses her research with Mulder, suspecting that L'Ively is the arsonist; this is confirmed by a police sketch taken from the witness' description. Upon reaching the Marsdens' house, the agents find the driver's charred body in the bathroom before the second floor bursts into flames. As Scully and Green escort the Marsdens to safety, Mulder faces his phobia and is able to save the Marsden children. Scully holds L'Ively at gunpoint but is forced to hold her fire when he informs her of the rocket fuel he has painted onto the house. Green throws a can of the rocket fuel in L'Ively's face, causing him to lose control and set himself alight outside.

With the case solved, Green returns to England with the Marsden family. Despite having sustained fifth and sixth degree burns over most of his body, L'Ively survives his immolation and is held in a medical facility as he awaits trial, healing at an alarming rate. The episode's final scene shows him asking a nurse for a cigarette.